The present invention relates, in general, to epitaxial growth on silicon, and more particularly to the growth of ordered nitride layers on silicon substrates.
There is a growing recognition of the benefits of producing nitride layers such as AlGaN or GaN structures on silicon substrates, instead of substrates such as sapphire, since silicon is inexpensive and readily available, has a higher thermal conductivity than sapphire, and is electrically conductive. Further, silicon substrates could be used in the production of conventional electronic devices and GaN-based devices side-by-side on a single wafer; however, reliable techniques for epitaxial growth of nitride on silicon have not been available. The growth of nitride layers on sapphire has been the subject of detailed investigation, but the details of epitaxial growth of nitride on silicon structures have not been fully investigated. One reason for this is that growth nucleation and formation of appropriate buffer layers on silicon structures have appeared to be difficult. Several approaches, including the use of intermediate layers, have been proposed to resolve the problems associated with epitaxial growth on silicon substrates, but none have been entirely satisfactory.
It is well understood that the problem of epitaxially growing nitride layers on silicon substrates arises from the uncontrolled formation of amorphous SiNx, on the silicon surface. This occurs, for example, when a gaseous nitrogen source is used during molecular beam epitaxial growth on a silicon substrate, for once the epitaxial chamber is exposed to nitrogen, or nitrogen containing gases or compounds, a nitrogen background is then present in the growth apparatus in subsequent growth experiments. In the case of a silicon substrate, such a nitrogen background results in the formation of a thin amorphous layer of SiNx at the surface of the silicon, once the wafer is heated to the growth temperature. The layer of SiNx, which is at most a few monolayers thick, forms spontaneously and is therefore difficult to control. This layer typically is not uniform, and the surface coverage it provides is typically in the form of islands. The average thickness of this layer depends on the background pressure of nitrogen, the temperature of the substrate, the exposure time, and details of the silicon surface structure. Even in a very clean system, i.e. with a low nitrogen background, formation of the Sixe2x80x94N bonds cannot be prevented.
One known approach to solving the problem of SiNx island formation is the use of low temperature nucleation, but this is at the cost of poor layer quality. The low quality of this nucleation layer can be improved by subsequent high temperature anneals, but no quantitative information exists as to the degree of recovery that can be obtained with this procedure. In addition, low temperature nucleation results in uncontrolled domain structure. Epitaxial layers of AlN and GaN grown on such nucleation layers are characterized by polarization domains at the surface, and this restricts the use of this material in device applications. In principle, a solution to this problem exists, for experience with the growth of GaAs on Si (001) shows that the propagation of one type of domains can be largely inhibited. While this approach has been successful in the growth of GaAs on Si(001) its use in the growth of GaN on silicon remains to be demonstrated.
A method of low temperature nucleation of AlN on Si(111) involving surface nitridation has been described by Kipshidze et al., Semiconductors 33(11), 1241 (1999). In this approach, the surface of Si was exposed at a low temperature to the nitrogen flux derived from a plasma source. MBE experiments on the growth of AlN on Si(111), carried out with plasma sources of active nitrogen, showed that growth could be initiated at temperatures lower than Tt=1100 K. Two dimensional (2D) growth appears more likely at low growth temperatures, where island nucleation density, rather than the adatom surface mobility, is optimized. However, this method does not produce high quality material.
A second approach to solving the problem of SiNx island formation is described in the literature as including the formation of a stable Alxe2x80x94Si phase, known as the xcex3-phase [Yasutake et al., J. Vac. Sci. Technol. A16(4), 2140 (1998)]. The xcex3-phase corresponds to the Al coverage of 1/3 ML (monolayer) where each atom of Al saturates three dangling bonds of Si. The detailed structure of this phase is not known. In the xcex3-phase, the atoms of Al are believed to saturate the surface bonds of Si and thus prevent nitridation. However, in the formation of AlN through the xcex3-phase, about 34% of the substrate surface area remains exposed to the nitrogen flux, with the resulting formation of Sixe2x80x94N bonds or amorphous SiNx. The resulting AlN will thus have at least two types of domains, one formed over Sixe2x88x92Al regions, the other on Sixe2x80x94N regions, and accordingly this method is not satisfactory.
The present invention overcomes the problems described above by providing a unique process for growing AlN and other nitrides on silicon. Briefly, the preferred embodiment of the invention includes first exposing a silicon wafer to a background flux of nitrogen, preferably in the form of ammonia, by placing the wafer in a molecular beam epitaxy (MBE) chamber. This produces a nucleated nitride layer on the silicon substrate thereby forming ordered Sixe2x80x94N bonds at the surface. A nitride-containing nucleation layer, preferably an AlN nucleation layer, is then formed on the surface of the substrate by subjected the wafer to a flux of aluminum using MBE, subjecting the wafer to a flux of nitrogen, also using MBE, and thereafter subjecting the wafer to additional alternate fluxes of aluminum and nitrogen. Finally, AlN is grown on the wafer by subjecting the AlN nucleated layer to a combined flux of aluminum and nitrogen using MBE. In particular, the nitrogen flux may be a gas containing ammonia. A GaN layer may then be grown on the silicon wafer, if desired. In this way, high quality epitaxial layers of nitride-containing compounds, such as AlN or GaN, can be grown on a Si(111) substrate by gas source molecular beam epitaxy (GSMBE) using NH3 as the nitrogen source.
More particularly, in the preferred embodiment of the invention the surface of a silicon substrate, or wafer, is first chemically etched to produce a hydrogen-terminated surface. The ordinary surface of Si, in the air, is covered by native oxides which may not be stoichiometric, i.e. their chemical composition may deviate from that of SiO2, and the presence of such oxides makes epitaxial crystal growth impossible. In the usual practice of molecular beam epitaxy, the native oxides are removed from the substrate surface by a chemical etch process, and once the native oxides are removed, the surface is covered with hydrogen. The resulting hydrogen-terminated surface is stable in the air for tens of minutes, thus providing the time needed to place the substrate in a conventional epitaxial growth apparatus. The sample is then heated to above 920 K, in a vacuum greater than 10xe2x88x928 torr, and the hydrogen leaves the surface. The hydrogen-terminated surface does not react with the background nitrogen or ammonia that may be present in the epitaxial chamber, up to a temperature of 920 K.
The second step to the process is to form the nitride nucleation layer, preferably an AlN nucleation layer, by first subjecting the etched substrate to background nitrogen, preferably ammonia, in the MBE growth chamber. The pressure of the background ammonia is maintained, with careful procedures, in the range of preferably 10xe2x88x927 to 10xe2x88x929 torr. The nucleation and growth temperature of AlN is preferably 1160xc2x130 K, which is dictated by considerations of surface kinetics. Since molecules of ammonia crack at the surface of a semiconductor substrate at temperatures in excess of 920 K, it is not possible to lower the growth temperature of the AlN. Within the typical constraints of the MBE apparatus, the surface of the Si substrate is exposed to ammonia for a few minutes, before the AlN nucleation or growth can be started. Even with the relatively low background pressure of ammonia achieved in the system, formation of Sixe2x80x94N bonds cannot be prevented.
The exposure of the silicon substrate to background nitrogen produces Sixe2x80x94N bonds on the surface of the substrate, resulting in a new 4xc3x974 surface structure, as seen by RHEED images. The period of the Sixe2x80x94N surface structure has an important relationship to the lattice constants of Si and AlN. Four periods of the Sixe2x80x94N related surface structure are almost equal to five periods of AlN. This is, to within 1%, the same relationship as that between Si(111) and AlN(0001). This relationship assures uniform and precisely oriented nucleation of AlN on the substrate of Si(111). In addition, once a layer of AlN is formed, this relationship results in complete relaxation of this layer through formation of misfit dislocations. Since a misfit dislocation starts at every fifth surface atom, dislocations are distributed uniformly on the surface.
In the next step of the process, an AlN nucleation layer is formed on the silicon substrate. MBE growth of AlN with a nitrogen source, preferably ammonia, at a range of growth temperatures of preferably about 1000-1200 K, is carried out. At temperatures below Tt (1100 K), AlN layers with qualities similar to the materials prepared using plasma sources of nitrogen have been obtained. However, it has been found that improved results are obtained when nucleation and growth of the AlN layer are carried out at temperatures above Tt, and preferably at temperatures of 1160xc2x130 K. After the silicon substrate reaches the growth temperature, it is subjected to a flux of aluminum, without ammonia and the Al atoms react with the surface nitrogen of the substrate. Reactions between nitrogen, aluminum, and silicon result in formation of a Sixe2x80x94Nxe2x80x94Al phase. Next, the aluminum flux is turned off and the substrate is subjected to a flux of ammonia. The silicon substrate is further subjected to an alternating process of aluminum flux only, followed by ammonia flux only. This repetitive process is continued until a flat surface is produced with full coverage of AlN. Once a flat structure of AlN is observed, AlN is grown on the substrate by subjecting the substrate to a combined flux of aluminum and ammonia.
The AlN layer which is formed using the foregoing gas source MBE provides a buffer layer which is needed for the further growth of GaN. High growth temperatures combined with controlled deposition of an AlN layer at the onset of epitaxy results in a very rapid transition to 2-D growth mode of AlN. Because of the formation of amorphous SiNx, at the surface of silicon, high quality epitaxial growth of GaN on Si had been quite difficult, as discussed above, for previously it was found that cracking could occur during the cool-down cycle due to the thermal expansion differences between GaN and silicon. However, because of the suppression of the formation of SiNx islands on the substrate in accordance with the present invention, and the assured uniform nucleation of a completely relaxed AlN layer at the Sixe2x80x94AlN interface, cracking of the GaN layer is eliminated. Triple crystal X-ray diffraction, transmission electron microscopy (TEM), Raman spectroscopy (RS) and photoluminescence (PL) have been used to show that the resulting nitride-silicon structures are similar to the best samples reported on sapphire.